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Many obstetricians already advise women to limit caffeine, though the subject has long been contentious, with conflicting studies, fuzzy data and various recommendations given over the years.

The new study, being published Monday in the Journal of Obstetrics and Gynecology, finds that pregnant women who consume 200 milligrams or more of caffeine a day — the amount in 10 ounces of coffee or 25 ounces of tea — may double their risk of miscarriage.

Pregnant women should try to give up caffeine for at least the first three or four months, said the lead author of the study, Dr. De-Kun Li, a reproductive and perinatal epidemiologist at the Kaiser Permanente Division of Research in Oakland, Calif.

“If, for whatever reason, they really can’t do it, think of cutting to one cup or switching to decaf,” Dr. Li said. “Stopping caffeine really doesn’t have any downside.”

Professional groups like the American College of Obstetricians and Gynecologists and the American Society for Reproductive Medicine have not taken official positions on caffeine, representatives said.

On Friday, the March of Dimes Web site said most experts agreed that the amount of caffeine found in 8 to 16 ounces of coffee a day was safe. It noted that some studies had linked higher amounts to miscarriage and low birth weight, but stated: “However, there is no solid proof that caffeine causes these problems. Until more is known, women should limit their caffeine intake during pregnancy.”

Now, having reviewed the new study, the March of Dimes plans to change its message, to advise women who are pregnant or trying to conceive to limit their daily caffeine intake to 200 milligrams or less, said Janis Biermann, its senior vice president of education and health promotion.

“Women do need good guidance,” she said.

Dr. Li’s study included 1,063 pregnant women who were interviewed once about their caffeine intake. At the time of the interview, their median length of pregnancy was 71 days. But 102 had already miscarried — not surprising, because most miscarriages occur very early in pregnancy. Later, 70 more women miscarried, for a miscarriage rate of 16 percent for the group — a typical rate.

Of 264 women who said they had used no caffeine, 12.5 percent had miscarriages. But the miscarriage rate was 24.5 percent in the 164 women who consumed 200 milligrams or more per day. The increased risk was associated with caffeine itself and not with other known risk factors like the mother’s age or smoking habits, the researchers said.

Dr. Li said the study answered an important question that previous research had left unresolved. Women who have morning sickness are less likely to miscarry than those who do not, possibly because the same hormonal changes that cause nausea and vomiting contribute to a healthy pregnancy. But some researchers said morning sickness could lead to confusing results in caffeine studies. These researchers argued that because they feel ill, some women may consume less caffeine. That tendency may make it appear that they are less likely to miscarry because they avoid caffeine, when the reason is actually that they began with healthier pregnancies.

Dr. Li said he and his colleagues had determined that the risk from caffeine was real and could not be explained away by different rates of morning sickness.

Dr. Carolyn Westhoff, a professor of obstetrics and gynecology, and epidemiology, at Columbia University Medical Center, had reservations about the study, noting that miscarriage is difficult to study or explain. Dr. Westhoff said most miscarriages resulted from chromosomal abnormalities, and there was no evidence that caffeine could cause those problems.

“Just interviewing women, over half of whom had already had their miscarriage, does not strike me as the best way to get at the real scientific question here,” she said. “But it is an excellent way to scare women.”

She said that smoking, chlamidial infections and increasing maternal age were stronger risk factors for miscarriage, and ones that women could do something about.

“Moderation in all things is still an excellent rule,” Dr. Westhoff said. “I think we tend to go overboard on saying expose your body to zero anything when pregnant. The human race wouldn’t have succeeded if the early pregnancy was so vulnerable to a little bit of anything. We’re more robust than that.”